Abstract: A rotary piston machine which has a substantially annular working space, which is divided by bars or the like dividing elements into a plurality of sealed chambers, the volumes of which are periodically changed. The bars or the like dividing elements are carried by two coaxial drums or pistons, which perform a differential rotational movement. The differential gear for the pistons comprises a bevel gear assembly, which comprises four bevel wheels, which mesh with each other. Two of said bevel wheels are coaxial with said pistons. The other two of said bevel wheels are centered on an axis which is at right angles to the axis of the pistons. The differential gear assembly also comprises an epicyclic gear or planet wheel assembly. One of said rotary pistons is operatively coupled by a shaft to one of said coaxial bevel wheels, and the other of said coaxial bevel wheels is freely rotatable relative to said shaft but is operatively coupled to the other of said rotary pistons.